This invention relates to a hydraulic brake booster with a booster piston, a pressure-control device, an annular piston bearing against the booster piston, and a central piston which is movable within limits relative to the annular piston by the brake pedal and actuates the pressure-control device, with the central piston and the annular piston being adapted to be coupled together after the force of a spring is overcome whereupon they are movable as one. A hydraulic brake booster of this type is known from German Pat. DE-OS No. 2,348,857.
In this known hydraulic brake booster, the central piston bears against an axially displaceable valve casing which controls a dual-seat valve. Exactly like all other components of the pressure-control device, this valve casing is pressure-balanced, which is expressly mentioned at the beginning of page 11 of the above-cited German patent.
From this feature it results that a predetermined brake pressure develops abruptly after a response force is overcome. This will be welcomed by the vehicle operator if a vehicle equipped with an automatic gearbox is to be prevented from rolling away by means of the brake pedal with the engine running idle while at standstill. However, if the brake is to be applied very sensitively, for example, on an icy road, there is the danger of the abrupt pressure buildup in the brake system causing lock-up of the brakes even before the range of the brake force becomes effective, i.e., a counter-force which is proportional to the brake pressure starts acting on the annular piston and, thus, on the central piston and the brake pedal.